Generation of vast quantities of waste material from sources such as municipal incinerators, nuclear reactors, nuclear weaponry manufacture and chemical industries present monumental problems for disposal. Land-fill is inappropriate for the most deleterious substances without prior processing to prevent leaching into the ground water. Radioactive material with a half-life of thousands of years presents the challenge of providing a permanent storage mechanism which substantially prevents release of radioactive contaminants into the environment.
Various techniques have been developed for vitrifying nuclear waste including a static melter at Savannah River Laboratories. Developments at the Savannah River Laboratories are described in the article "Application Of High Level Waste-Glass Technology To The Volume Reduction And Immobilization Of Tru, Low Level, And Mixed Wastes" by Bickford, D. F., NSRC-MS-90-274.
In a prior application of Ray S. Richards, Ser. No. 510,556 filed on Apr. 18, 1990, one of the applicants herein, a high-speed mixer/melter was disclosed in which a shaft rotates an impeller at high speeds in a melt contained in a vessel. Electrical energy is simultaneously discharged through the melt between the walls of the vessel and the impeller. One problem encountered with the previous mixer/melter designs related to sealing the cover to prevent the escape of gases from the vessel. Providing an elastomeric seal on the cover of a vessel operating in excess of 1,000.degree. C. will only result in the melting of the seal. Providing mechanical seals of an electrically conductive material could establish an electrically conductive path and result in a short circuit between the impeller shaft and the walls of the vessel. An alloy having excellent corrosion resistance, such as alloy 690, is preferably used to construct the vessel, including the vessel cover.
Axial adjustment of the location of the impeller by moving the shaft axially relative to the cover presents additional sealing problems. It may be necessary to adjust the height of the impeller in the vessel depending upon the density of the output required, quantity of melted material contained in the vessel, and the like.
Another problem relating to off-gas porting from the vessel relates to maintaining a clear port through which hot gases may be removed without blocking the port as gases cool upon removal from the vessel. A film cooler was proposed in "Off-Gas System For The Savannah River Plant Defense Waste Processing Facility" by Randall, C. T. et al., in which steam is injected through angled slots formed in a sleeve through which off-gases are ported. The film cooler allows off gases to cool while preventing the solidification of condensed gases on the walls of the off-gas port. One problem encountered by this device relates to the difficulty in removing the film cooler from the vessel for service or replacement. Another problem presented by the design proposed by the Randall article is the large volume of coolant fluid comprising air and steam, which is required. In addition, the cost of fabricating a film cooler having machined slots for directing the fluid is substantial.
Another problem faced by prior art hazardous waste melters related to providing a bottom drain which is selectively sealable and provides a redundant sealing mechanism whereby the flow of molten material may be prevented by freezing such flow.
Another problem presented by certain waste streams is the need to provide a chamber for superheating molten material to above 1,100.degree. C. for a period of time to dissolve any crystalline material remaining in the melt after processing in the high-speed mixer/melter. At such extreme high temperatures, problems relating to durability of a tank lined with an alloy are encountered. Ensuring adequate holding time for molten material contained in the vessel presents additional problems.
Additional problems encountered by prior art melters are solved by the present invention as summarized below.